This book constitutes the thoroughly refereed post-proceedings of the Fourth International Workshop on Formal Aspects in Security and Trust, FAST 2006, held in Hamilton, Ontario, Canada, August 26-27, 2006.The 18 revised papers presented were carefully reviewed and selected from 47 submissions. The papers focus of formal aspects in security and trust policy models, security protocol design and analysis, formal models of trust and reputation, logics for security and trust, distributed trust management systems, trust-based reasoning, digital assets protection, data protection, privacy and ID issues, information flow analysis, language-based security, security and trust aspects in ubiquitous computing, validation/analysis tools, web service security/trust/privacy, GRID security, security risk assessment, and case studies.